The present application is a continuation in part of U.S. Pat. application Ser. No. 036,287 filed Apr. 9, 1987 and entitled "Materials Preparation Technique". The disclosure of '287 application is hereby incorporated by reference herein.
As discussed in greater detail in the '287 application, materials known as Chevrel-phase materials, also called Chevrel compounds, include an invariant portion which may consist essentially of molybdenum and a chalcogen such as sulfur, selenium, tellurium, or mixtures of these with or without minor amounts of oxygen. Ordinarily, the fixed portion has a stoichiometric formula of Mo.sub.6 Z.sub.n where Z represents the chalcogen and n is usually between about 7.5 and about 8.5, most typically about 8. The unique crystal structure of the materials permits intercalation of metals, so that the overall stoichiometry of the Chevrel-phase material can be represented as A.sub.x Mo.sub.6 Z.sub.n where A represents the intercalated metal and x may vary from 0 (no intercalated metal) to an upper limit which may be about 4 or less depending upon the particular metal.
Energy is released upon intercalation of the metal into the Chevrel-phase material and as the intercalation process is partially or wholly reversible with certain metals, the Chevrel-phase materials can be used as cathodes in electrochemical cells. As described in Mulhern et al., "Rechargable Non-Aqueous Lithium/Mo.sub.6 S.sub.8 Battery", (Can. J. Phyz., Vol. 62, pp. 527,631, 1984) a cell with a lithium anode and a Chevrel-phase cathode of the formula Li.sub.x Mo.sub.6 S.sub.8 can be subjected to a charge cycle in which lithium is removed from the Chevrel-phase by the applied electrical energy. In a discharge cycle, the lithium is re-intercalated into the Chevrel-phase releasing energy as electrical energy.
Lithium intercalated Chevrel-phase material can be made by first preparing copper intercalated Chevrel-phase Cu.sub.x Mo.sub.6 S.sub.8 by direct reaction between copper, molybdenum and sulfur at elevated temperature and then leaching the copper from the material to prepare the un-intercalated Chevrel-phase Mo.sub.6 S.sub.8. Lithium is then intercalated into the so-formed Mo.sub.6 S.sub.8 either chemically or electrochemically. In the latter, the cell as assembled has a cathode of copper intercalated Chevrel-phase material and a lithium anode. On the first discharge cycle, lithium displaces the copper to yield the lithium intercalated material. Likewise, lithium can replace iron or nickel in the corresponding iron intercalated or nickel intercalated Chevrel-phase materials.
One significant aspect of the invention discussed in the '287 application includes the discovery that the lithium-intercalated Chevrel-phase materials can be synthesized directly. Thus, lithium-intercalated cathodes for storage cells can be made without the leaching or controlled discharge step formerly required.
As further disclosed in the '287 application, the reaction mixture containing lithium, molybdenum and sulfur for direct formation of the lithium-intercalated Chevrel-phase materials can be prepared by heating a precursor mixture including a heat-labile lithium compound together with molybdenum and sulfur, typically as a mixture of MoS.sub.2 and free Mo. Upon heating, the heat-labile compound yields volatile decomposition products which may be swept from the mixture, as by a stream of inert gas, leaving behind the lithium, molybdenum and sulfur to form the Chevrel-phase material. This approach greatly simplifies the synthesis inasmuch as it avoids the difficulties encountered in dealing with free metallic lithium and permits the use of relatively inexpensive starting materials which can be handled without special precautions.
As reported in Tarascon et al., "Electrochemical, Structural and Physical Properties of the Sodium Chevrel-phases Na.sub.x Mo.sub.6 X.sub.( 8-y)I.sub.y ", J. Solid State Chem., Vol. 66, pp. 208.div.224, 1987, the intercalation of sodium into Chevrel-phase materials such as Na.sub.x Mo.sub.6 S.sub.8 is only partially reversible, so that sodium can be removed and re-intercalated over the range x=1 to about x=4. As further reported in the Tarascon et al. article, the sodium intercalated Chevrel-phase materials can be employed with a sodium anode to form a secondary or rechargeable storage cell. The sodium Chevrel-phase materials used in the Tarascon et al. article were prepared by first preparing a copper intercalated material, leaching the copper to provide Mo.sub.6 S.sub.8 and then conducting a controlled intercalation of the sodium into the un-intercalated material in the electrochemical cell. As will be appreciated, these are essentially laboratory scale procedures which are not suitable for industrial production.
Despite the foregoing developments, however, there have still been needs for further improvements in electrochemical cells incorporating Chevrel-phase cathodes and in synthesis methods for Chevrel-phase materials.
This is because the discharge characteristics are dependent on the cathode material and in particular, for Chevrel-phase materials, on the actual composition of the material. thus, although some Chevrel-phase materials show considerable change of cell voltage during the discharge, it is possible for example, to select those which are capable of providing good cell capacity with almost no decline in voltage during discharge. For some applications this is a highly desirable feature.